There are particularly onerous requirements for an insulating material for an underwater flowline. A material containing microspheres should be resistant to their breakage during the preparation of the material and subsequent application to a pipe. The material should also permit substantial tensile elongation, exhibit low creep over a long period, perhaps a service life of 30 years during which hot oil may be conveyed within the flowline, and have excellent mechanical properties.
Polyolefin/filler composite materials in which the filler comprises "glass bubbles" are described in Research Disclosure 253015. This article discloses that it is possible to compound "glass bubbles" with, inter alia, polypropylene using extrusion equipment, with less than 5% by weight breakage to produce low-density composite materials. Proposed uses thereof are extruded parts for buoyancy and thermal insulation and large automotive moldings.
When it was attempted, during experimentation, to incorporate microspheres into a polyolefin to make a composite material having properties suitable for rigorous applications such as flowline insulation, it was found to be possible to do so adequately only by using base polypropylenes of high Melt Flow Index (MFI). The resulting composite materials generally had MFI values which were too high for application by the "pipe extrusion" method and/or had unsuitable mechanical properties for flowline insulation. When it was attempted to incorporate microspheres in a base polypropylene of low MFI, a substantial proportion of the microspheres were broken during the compounding or subsequent extrusion, so that the insulation properties were inadequate.
EP-A-0473215 discloses a process of producing a composite material which can be employed in rigorous application areas, for example as an insulating casing for an underwater flowline that can be applied by coextrusion with a pipe. The process comprises treating at least some of the microspheres to be incorporated in the polyolefin with a chain-scission agent; distributing the microspheres evenly in the polyolefin in its melt phase; and forming the composite material into a desired shape and cooling.
EP-A-0473215 includes specific examples of test pieces that are satisfactory for pipe extrusions, although no such extrusions are exemplified. EP-A-0473215 also suggests that the composite material can be extruded in the form of tapes and applied by "side-wrapping" to a steel pipe as that rotates.
During fabrication of an insulated flexible flowline, the insulation material which is applied in a tape form experiences severe tensile forces. To adequately meet these forces, dependent on the pipe diameter and tape dimensions, elongation at break of 1% to 40% or more is needed. Further, the surface of the tape should not have any notches or defects as these cause the tape to break during winding. Further, the tape needs to be able to be manufactured in continuous lengths of several km.
In addition, during service of an insulated flowline, the insulation material should have an adequate thermal conductivity which may be derived from the presence of intact hollow glass microspheres. It is imperative that both during the manufacture of the syntactic foam tape and during the fabrication of the insulated flowline the microspheres do not break.
Further, as indicated above, the insulation material should maintain the insulating properties during the service life (10 to 30 years) by withstanding high hydrostatic compression and creep.
It is known from polypropylene extrusion that, to obtain a smoother surface finish, die temperatures higher than normal (210.degree. C.) and longer land length are needed (i.e., relatively high ratio of die land length 1 to die thickness t.). Higher temperatures should also minimize microsphere breakage. However, when this conventional wisdom was applied to materials of the type described above, tapes and melts of unacceptable shark-skin effect resulted. The shark-skin effect is less relevant when using the pipe extrusion technique (as is also flexibility), and will in any event usually be covered by an outer plastics layer (produced by coextrusion).